1. Field of the Invention
This invention relates to a device for recording or playing back video and data signals on or from a rotary recording medium such as a magnetic disk. More particularly, it relates to a high speed, accurate recording or playback head positioner.
2. Background Art
Recently developed electronic still cameras are designed to convert the still images of objects formed through lenses into video images by means of solid-state pickup elements, record the images on magnetic disks, and play back the images on separately installed television systems. Alternatively hard copies can be obtained by using printers.
In such a camera system, a magnetic disk about 5 cm in diameter is used and, as shown in FIG. 53, 50 recording tracks are concentrically formed thereon, each recording track band being 60 micrometers in width and each guard band between tracks being 40 micrometers in width.
The first photograph which is taken after a new disk is mounted on an electronic still camera, i.e., the first still image, is normally recorded in a first track position TR.sub.1 which is outermost on a disk recording surface 10a. At that time, a recording head 12A shown in FIG. 1, arranged opposite to the disk recording surface 10a is moved from a fixed home position HP (see FIG. 53) to the first track position TR.sub.1 and a signal which covers one field and which is equivalent to the still image is written to the whole circumference in the first track position TR.sub.1 of the disk 10, which is rotating at a fixed speed of 3,600 rpm. When the first recording track is thus formed, the recording head 12A is kept at that first track position TR.sub.1 on standby or returned to the home position HP.
When a second still image is recorded in a second track position TR.sub.2 located one track (assuming a track pitch P.sub.t of 100 micrometers) toward the inner side of the first track position TR.sub.1, the recording head 12A is moved from the first track position TR.sub.1 or the home position HP to the second track position TR.sub.2 and a field signal is written to the second track position TR.sub.2 in the same manner as described above.
The recording head 12A is thus moved to a track position TR.sub.n where a field signal is written each time a still photograph is taken and, for this reason, utmost accuracy is required for such positioning. Unless the recording head 12A is accurately positioned, an actual track tr.sub.n will be formed in a position deviating from a corresponding track position TR.sub.n and it will be impossible to accurately access the track at the time of playback. In the worst case, the actual track and the adjacent track tr.sub.n-1 or tr.sub.n+ 1 may overlap and this will allow data to be recorded without forming a guard band. In this case, the playback head can scan the adjacent recording track tr.sub.n-1 or tr.sub.n+ 1 when the data is being read from the recording track tr.sub.n, thus causing crosstalk.
Conventionally, a step motor is used to move and position the recording head 12A, e.g., the step motor responding to command pulses is so arranged as to rotate an angle of 1.5 revolutions and send the recording head 12A by 4.0 micrometers in the direction shown by arrow F.sub.i or F.sub.o of FIG. 5. In that case, if 25 command pulses are continuously given by a control means to the step motor when the recording head 12A is located in the n-track position TR.sub.n, the recording head 12A will be moved about 100 micrometers (and track pitch in the direction of arrow F.sub.i and positioned in the adjacent track position TR.sub.n+1. If the step motor is excited opposite, in an opposite manner the recording head 12A will be moved in the outward direction of arrow F.sub.o and positioned in the opposite adjacent track position TR.sub.n-1. When the recording head 12A is moved from the home position HP to a specified track position, e.g., TR.sub.n, command pulses equivalent to the moved distance are given by the control means to the step motor. The movement and positioning of the recording head 12A are thus controlled by the number of command pulses given by the controller to the step motor and the present position of the recording head 12A. is also monitored by the controller on the basis of a cumulative value of the command pulses given until then.
In the conventional head positioner of the above recorder, an open loop control system is employed to position the head by means of the step motor, whereby the actual head position will not correspond to what is specified by the command pulses because of the presence of backlash if the transfer elements between the step motor and the recording head support (such as gears) generate backlash and a deviation. The disadvantage is that the deviation of the recording track from the format track position poses the above-described access problem at the time of playback. The deviation of the recording track resulting from the mechanical error of the positioner not only degrades compatibility with the playback but also causes crosstalk at the time of playback even with the application of a tracking servo because the transfer error attributed to the backlash increases while the recording head is being moved from the first track position to the second track position and then the third track position, thus ultimately allowing the field signal which should be recorded in the n-th track position TR.sub.n to overlap by being recorded in the adjacent (n-1)st track position TR.sub.n-1 or the (n+1)st track position TR.sub.n+ 1 (without guard band formation).
The tracking servomechanism at the time of playback is designed to position the playback head right in the center of the recording track so that it can accurately scan the latter and the conventional servomechanism is often used to control the so-called singal envelope. Control of single envelope is disadvantageous in that high speed access or tracking is unattainable because the playback head is moved gradually toward the center position of the recording head. An additional problem is that the actual head position will never be superposed on what is instructed by the command pulses if backlash occurs, so that accurate head transfer will become impossible.